PBX telephone systems are well known for providing switched communications between a plurality of PBX subscriber telephones, computers, etc. The switched communications take place through a switching matrix. The matrix makes the actual signal interconnect between the PBX user signal ports, at which the subscriber equipment (i.e. station equipment such as telephones, computers, etc.) is connected. In modern digital PBX's the signal information (voice, data, control) is typically in a byte wide, pulse code modulated (PCM) format, and the matrix is a time slot interchange (TSI) which time division multiplexes the PCM sample bytes between ports.
The signal flow through the matrix, i.e. the ports to be connected/disconnected, is controlled by the PBX call processor. The call processor is the signal traffic control authority in the system. It detects the change in port status from idle to active, the dialed number from the originating port which identifies the desired destination (interconnect) port, and termination of communications between ports. In each instance, it send the commands to the matrix and to the other PBX functions which support the communications between ports. Fundamentally the call processor provides the basic control, while the other support functions enhance user operation of the system.
The support functions are generally those required to make the PBX user friendly to the user. This includes generation of "busy signals" when a dialed destination port is in use (off-hook), a "ring back signal" when a dialed port is idle (on-hook), DTMF dial, etc. All designed to provide feedback to the human operator. In addition there is the "conferencing" of calls between three or more user ports simultaneously.
Other of the support functions improve overall PBX operating efficiency and reliability. These include: modems for connecting PCM type PBXs to analog trunk lines, signal error rate monitoring, universal asynchronous receiver-transmitter (UART), and signal data communications code conversion.
In the prior art PBX systems, the support functions are distributed in various dedicated circuits, i.e. different service circuit cards installed in the PBX central equipment. While some of these support functions/features are optional, i.e. not necessary to basic service, the PBX system must accommodate the different dedicated circuitry for optional service. This requires several different service circuit cards. Even more significant, is the requirement to inventory all of the different types of service circuit cards of a given PBX system to ensure availability of replacements for customer support.